What is pertinent is that he has done damage to a brand after his contract to act as its ambassador was over.

The market for brand ambassadorships has suddenly got a jolt, after Amitabh Bachchan's reported reflections on Pepsi, made in the course of telling students at the Indian Institute of Management, Ahmedabad, that it was important to carry out due diligence on a brand before undertaking to endorse it.

Bachchan said a young schoolgirl in Jaipur had asked him why he was promoting something that her teacher had said was poison. This made him realise that the brand he promoted affected another brand that he was even more intimately associated with: himself.

So, he advises his son and daughter-in-law to be careful about the brands they promote, he said.

Now, it is plausible that Bachchan did not explicitly say that he ended his contract with Pepsi because he heard it out of the mouth of babes that the stuff was poison, that he merely spoke in a manner that allowed the audience to infer a linkage between his ceasing to endorse Pepsi and the little girl's toxic remark. The precise manner in which he brought harm to a brand that he has endorsed in the past does not matter.

What is pertinent is that he has done damage to a brand after his contract to act as its ambassador was over. This harms not just the brand in question but the entire specialised line of business called brand ambassadorship.

If it becomes acceptable for a celebrity to trash a brand after his contract to celebrate it is over, rival brands could pay to make sure that such trashing does, indeed, take place. This would destroy the market for brand ambassadorship altogether.

Bachchan is too much of a professional not to appreciate this. The solution is for brand ambassadorship contracts to make it legally binding, on pain of serious damages, for celebrities to follow a policy of do-no-harm even after their active promotion phase is over.

The media could also play a useful role to reinforce discipline: minute scrutiny of contra-brand behaviour would make celebrities mindful of the potential damage to their own public image. That would move brand endorsement from the merely commercial realm to an existential one.